Solution based enhancements of fuel cell components and other electrochemical systems and devices

ABSTRACT

This invention relates in general to components of electrochemical devices, and to methods of preparing the components. The components and methods include the use of a composition comprising an ionically conductive polymer and at least one solvent, where the polymer and the solvent are selected based on the thermodynamics of the combination. In one embodiment, the invention relates to a component for an electrochemical device which is prepared from a composition comprising a true solution of an ionically conductive polymer and at least one solvent, the polymer and the at least one solvent being selected such that |δ solvent−δ solute|&lt;1, where δ solvent is the Hildebrand solubility parameter of the at least one solvent and where δ solute is the Hildebrand solubility parameter of the polymer. In another embodiment, the invention relates to a method of improving at least one property of a component for an electrochemical device or at least one property of the electrochemical device, the method comprising preparing the component from a composition comprising a true solution of an ionically conductive polymer and at least one solvent, the polymer and the at least one solvent being selected such that |δ solvent−δ solute|&lt;1, where δ solvent is the Hildebrand solubility parameter of the at least one solvent and where δ solute is the Hildebrand solubility parameter of the polymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage of International Application No.PCT/US2005/041571 filed Nov. 16, 2005, the disclosures of which areincorporated herein by reference, and which claimed priority to U.S.Provisional Application No. 60/628,834, filed Nov. 16, 2004, and U.S.Provisional Application No. 60/628,797, filed Nov. 16, 2004, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates in general to electrochemical systems anddevices, and in particular to fuel cell components.

A fuel cell is an electrochemical “device” that continuously convertschemical energy into electric energy (and some heat) for as long as fueland oxidant are supplied. Fuel cells are evolving. Some currently knowncategories of fuel cells include polymer electrolyte membrane (PEM),alkaline, phosphoric acid, molten carbonate, solid oxide, and biobased.All of these fuel cell types have the advantages of silent operation,high efficiency and zero emission capability. PEMs, however, offerseveral distinct advantages over the others. Some of these are lowtemperature operation (80-150 C), quick-start-up, compactness, andorientation independence.

At the heart of the PEM fuel cell is a membrane that has thin coatingsof catalyst applied to both sides comprising a membrane electrodeassembly (MEA). As hydrogen flows through the anode side of the MEA, aplatinum-based catalyst facilitates the disassociation of the hydrogengas into electrons and protons (hydrogen ions). The hydrogen ions passthrough the thickness of the membrane and combine with oxygen andelectrons on the cathode side, producing water and heat. The electrons,which cannot pass through the membrane, flow from the anode to thecathode through an external circuit containing an electric load, whichconsumes the power generated by the cell. FIG. 1 shows a detailedschematic of a PEM fuel cell.

Fuel cells have been around since 1839, but they have been hindered bycomponent materials which are high in cost and suffer from poordurability. Nevertheless, they have attracted much interest in recentyears for their ability to produce electricity and heat with higherefficiency and lower emissions than conventional energy technologies.However, the cost of fuel cells is still too high and technicalbreakthroughs are required before broad commercial application canbecome a reality.

Despite recent advances in the design of fuel cell components, furtherimprovements are required to transform fuel cells from the fundamentalsciences into enabling technologies.

SUMMARY OF THE INVENTION

This invention relates in general to components of electrochemicaldevices, and to methods of preparing the components. The components andmethods include the use of a composition comprising an ionicallyconductive polymer and at least one solvent, where the polymer and thesolvent are selected based on the thermodynamics of the combination.

In one embodiment, the invention relates to a component for anelectrochemical device which is prepared from a composition comprising atrue solution of an ionically conductive polymer and at least onesolvent, the polymer and the at least one solvent being selected suchthat |δ solvent−δ solute|<1, where δ solvent is the Hildebrandsolubility parameter of the at least one solvent and where δ solute isthe Hildebrand solubility parameter of the polymer.

In another embodiment, the invention relates to a method of improving atleast one property of a component for an electrochemical device or atleast one property of the electrochemical device, the method comprisingpreparing the component from a composition comprising a true solution ofan ionically conductive polymer and at least one solvent, the polymerand the at least one solvent being selected such that |δ solvent−δsolute|<1, where δ solvent is the Hildebrand solubility parameter of theat least one solvent and where δ solute is the Hildebrand solubilityparameter of the polymer.

Various other embodiment of the invention are described in the followingspecification, claims and/or drawings.

Some advantages of this invention will become apparent to those skilledin the art from the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic diagram of a conventional PEM fuelcell.

FIG. 2 is a schematic view of an electrospinning setup.

FIGS. 3 and 4 are scanning electron micrograph images of ionomer fibersproduced by an electrospinning process. FIG. 4 is an enlarged view of aportion of the image in FIG. 3.

FIG. 5 is a typical polarization curve for a PEM fuel cell.

FIG. 6 is a representative polarization curve of an electrode accordingto the invention compared to a state of the art electrode.

FIG. 7 is a close-up of an activation polarization region comparing anelectrode according to the invention with a state of the art electrode.

FIG. 8 is a three-phase interphase schematic for a single catalytic siteat the cathode of a PEM fuel cell.

FIG. 9 is a field-emission SEM image of a Pt/C electrode.

FIG. 10 is a graph comparing the durability of an electrode according tothe invention with a state of the art electrode.

FIG. 11 shows AFM tapping mode images of electrodes fabricated fromn-butyl acetate according to the current state of the art.

FIG. 12 shows AFM tapping mode images of electrodes fabricated fromt-butyl alcohol according to the present invention.

FIG. 13 is a representative polarization curve comparing one MEA with ahigh aspect ratio ionomer fiber (according to the invention) and onewithout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention uses a novel solution thermodynamics approach toselecting an ionically conductive polymer and at least one solvent forpreparing a component of an electrochemical device, such as a fuel cell.The thermodynamic approach is described in more detail below.Advantageously, the component prepared according to the invention mayinclude improvement(s) compared to components prepared by conventionalmethods.

For example, the present invention may enhance fuel cell performanceand/or durability by engineering the three-phase interphase of the MEAsthrough 1) the formulation of novel ionomer binder solutions; and,optionally, 2) the development of novel, high aspect ratio ionomerfibers to be used as precursors for electrode and/or MEA fabrication.The term “high aspect ratio”, as used herein, means fibers having anaspect ratio within a range of from about 100:1 to about 1000:1(length:diameter). The invention may create an optimized,multifunctional, nanostructured architecture which reduces polarizationlosses (reaction rate, resistance and mass transport losses) and/orcatalyst loadings. The terms “ionomer” and “ionically conductivepolymer” are used interchangeably herein to refer to a polymer havingany significant proportion of ionizable and/or ionic groups.

The dominant polarization losses in a hydrogen-air fuel cell are due tothe poor kinetics of the oxygen reduction reaction (ORR) and theresulting transport limitations of the protons and reactants at thecathode. An understanding of the interactions at the three-phaseinterphase has allowed the development of high-performance electrodesand MEAs that may reduce the polarization losses, while increasingefficiencies, energy densities and durability. The experimental detailsregarding MEA fabrication, single-cell testing and characterization(i.e. cyclic voltammetry and atomic force microscopy) are presentedhereinbelow.

The components prepared according to the invention are important to fuelcells because they may cross all applications, fuels and chemistries.They also may have applications beyond fuel cell technologies, such asionic polymer metallic composite (IPMC) actuators/sensors. Further, whencatalysts are referred to herein, it should be recognized that theinvention is not limited to catalysts per se, but it may also beapplicable more generally to metals/inorganics (including salts, oxidesand metal alloys) which may or may not facilitate the electrochemicalreaction.

Since the focus of this invention is on the enhancement of componentsderived from two different MEA fabrication techniques, 1) theformulation of a novel ionomer binder solution and 2) the development ofnovel, high aspect ratio ionomer fibers to be used as precursors forelectrode and MEA fabrication, the following description is devoted todetailing the membrane electrode assembly fabrication of each.

Ionomer Binder Solution Formulation. The invention includes a novelsolution thermodynamics approach to select an ionomer and at least onesolvent for use in a composition to prepare a component of anelectrochemical device, such as an ionomer binder solution used toprepare an electrode. The composition is a true solution, not adispersion or a colloidal suspension. The true solution is a singlephase. A dispersion would consist of at least two phases with aninterface between the dispersed and continuous phases.

According to this solution thermodynamics approach, the change in freeenergy, ΔG, upon mixing the ionomer and the solvent(s) must be negativefor the solution to be thermodynamically feasible.

ΔG=ΔH−TΔS; <0 for solubility, where ΔH is the change in enthalpy and TΔSis the product of temperature, T, and the change in entropy, ΔS.

Analyzing the above equation, it can be determined that ΔH drivespolymer solubility due to specific interactions since TΔS is generallylow for polymers due to the low amount of configurational possibilities.

ΔH is related to the Hildebrand solubility parameter, δ, where |δsolvent−δ solute|<1 for solubility. The results of the equation couldalso be <0. This parameter represents the total van der Waals force.There are three types of interactions that are most commonly used insolubility theory. They are dispersion forces (induced dipole-induceddipole or London dispersion forces), polar forces (dipole-dipole forces)and hydrogen bonding forces. The Hildebrand value for a solvent mixturecan be determined by averaging the Hildebrand values of the individualsolvents by volume.

In addition to the considering the Hildebrand solubility parameter, insome cases to optimize the invention it may be desirable to considerother solvent properties, such as boiling point/vapor pressure,evaporation rate, surface tension, viscosity, hydrogen bonding,dielectric constant and/or dipole moment.

Diffusion may be enhanced by varying the molecular weights andcomposition of the ionomer binder materials. For example, low molecularweight ionomer binders will tend to have increased solvent diffusionrates and H₂/O₂ permeation rates to electrocatalyst sites. Anotherexample would be utilizing materials that have inherently highdiffusivity/permeation to H₂/O₂, such as PTFE, PVDF, Nafion®, blends andside-chain chemistries.

Further ways to enhance electrode performance may include manipulatingthe ionic functionality, hydrophobicity and/or porosity of the electrodefor improved water management (to reduce mass transport losses) whileoptimizing the triple phase boundary (three-phase interphase) where thehydrogen oxidation reaction and the oxygen reduction reaction can onlyoccur at localized regions where electrolyte, gas and electricallyconnected catalyst regions contact. One way to accomplish this isthrough various additive technologies. These could be fluorine-based orinorganic additives (e.g. heteropoly acids, zirconium phosphate, etc.).

Two different catalyst ink formulations were prepared. Each contained astandard catalyst: Nafion® ratio of 2.5:1 (28 weight percent ionomerbinder) using 5 weight percent Nafion® 1100 EW solution (ElectroChem,Inc.) and 20 weight percent Pt on Vulcan XC-72 carbon black (E-tekDeNora). To one formulation, t-butyl alcohol (Aldrich) was added as adiluting solvent, and, to the other, n-butyl acetate (Aldrich) wasadded. Formulations were allowed to stir on a stir plate overnight atroom temperature. The added weight of these chemicals was equal to theweight of the Nafion® binder solution in each formulation. In someinstances, sonication was used to aid in the dispersion of theelectrocatalyst particles. It should be noted that varying ionomerbinder loadings (i.e. catalyst: ionomer binder) could be used.

Five (5) cm² transfer decals were prepared from glass-reinforcedpolytetrafluoroethylene (PTFE) films (Saint-Gobain). Each catalyst inkcoat/layer was painted on one side of each decal with a flat brush(Winsor Newton) with the appropriate catalyst ink under infrared heat toa final dry weight containing ˜0.2 mg catalyst per cm² electrode.

Nafion® 112 (Aldrich) was converted to the salt form, then MEAs werefabricated using the following procedure on a Carver hydraulic press:(1) Place MEA assembly into pre-heated (210 C) press and compress at 400psig for 10 minutes. Note: This temperature and pressure may be higherdepending on the type of membrane and binder material. (2) Cool underpressure to room temperature. (3) Remove from press. (4) Peel decalsaway from the MEA assembly one at a time leaving only the electrodesfused to the membrane.

The ionically conductive polymers for use in the invention may be anythat are currently known or developed in the future. Some generalcategories of ionically conductive polymers may include the following.Canonical: Nafion®—poly(TFE-co-perfluorosulfonic acid). A sulfonatedversion of almost any poly(aromatic), such as Radel®, Kraton®, PBI, etc.Other acid groups applied to the above: sulfonimides, phosphonic acids,etc. Supported versions of the above: Gore-TEX® etc. used as supports.Polymers with imbibed solid or liquid acids, such as PBI/phosphoric acid(CWRU®) or phosphotungstic acid.

For example, some specific examples of polymers that may be used in theinvention are taught in PCT App. No. PCT/US01/29293, filed Sep. 21,2001, entitled “Ion-Conducting Sulfonated Polymeric Materials”, and thepreferred materials are, in particular, BPSH-xx (Bi Phenyl Sulfone) and6F-XX-BPSH-XX (Hexafluoro Bi Phenyl Sulfone) which were used asdescribed herein. As well, other polymers that may be used in thepresent invention are taught in PCT App. No. PCT/US03/09918, filed Apr.1, 2003, entitled “Sulfonated Polymer Composition for Forming Fuel CellElectrodes” PCT App. No. PCT/US03/03864, filed Feb. 6, 2003, entitled“Polymer Electrolyte Membranes Fuel Cell System”; and PCT App. No.PCT/US03/03862, filed Feb. 6, 2003, entitled “Polymer ElectrolyteMembranes for Use in Fuel Cells”. Other polymers that may be used aredisclosed in U.S. Pat. No. 6,670,065 B2, issued Dec. 30, 2003, U.S. Pat.No. 6,893,764 B2, issued May 17, 2005, and U.S. Patent ApplicationPublication No. 2005/0031930 A1, published Feb. 10, 2005. Furtherpolymers that may be used are disclosed in U.S. Provisional ApplicationNo. 60/736815, filed Nov. 15, 2005, entitled PEM's for Fuel CellApplications. The above-mentioned applications and patents areincorporated herein by reference, as if fully set forth herein. Thepresent invention may be advantageously used with the materialsdescribed therein, which include the materials referred to asBattellion™.

The solvent(s) for use in the invention may be any that are currentlyknown or developed in the future that are suitable for preparingcomponent(s) of electrochemical devices such as fuel cells. Someexamples of typical solvents and diluting agents (co-solvents) used forboth Nafion® and non-Nafion® (i.e. hydrocarbon) ionomers are shown inTable 1.

High Aspect Ratio Ionomer Fiber Development. First, a BPS45 ionomer truesolution (Na⁺ form) was formulated in N,N-dimethyl acetamide (DMAC) to˜35 weight percent concentration. This solution was characterized asfollows: surface tension, γ=40 dynes/cm; conductivity, σ=1574 μS/cm;viscosity, η=0.019 Pa·s @ 10³-10⁴ s⁻¹.

The polymer(s) and solvents(s) used in this embodiment of the inventionmay be the same as those described above, or they may be different.

This solution was then loaded into a 10 ml glass syringe (Popper & Sons,Inc.) with either an 18 or 20 gauge syringe needle. The tip of thisneedle can vary (e.g. ball, blunt/straight, etc.). This syringe andneedle were placed into an electrospinning setup as shown in FIG. 2.

The parameters for spinning the ionomer fiber were as follows: θ˜20-40°,d=4-12 cm, and potential of 10-25 kV. Electrostatic processing commencedonce the gravitational forces caused the ionomer solution to exit thetip of the needle. Upon droplet formation, the high-voltage, directcurrent power supply was activated to form a Taylor cone, and theresulting ionomer fiber was electrospun into a non-woven mat, which wascollected on the grounded target. SEM images of the ionomer fibers areshown in FIGS. 3 and 4.

This setup could be modified by laying the syringe in a horizontalposition and using a syringe pump to deliver the ionomer solution to thesyringe needle tip. The target would also be relocated so that it wouldbe perpendicular to the syringe needle. This setup could be furthermodified with vacuum-assist if higher boiling point solvents, such asN-methyl-2-pyrrolidone (NMP), are used.

The fibers produced according to the above-described method are usuallycontinuous fibers. It is also believed that non-continuous fibers may beelectrospun and produced in accordance with the present invention toproduce a mat, using techniques such as are taught in U.S. Pat. No.6,252,129, issued Jun. 26, 2001, to Coffee (incorporated by referenceherein).

The catalyst ink formulation was prepared as discussed previously usingt-butyl alcohol as a co-solvent.

Single MEAs could be fabricated primarily by two different techniquesutilizing this technology. One would be spinning the fiber directly ontothe electrode described earlier, which also serves as the groundedtarget, and then applying a PEM. Another technique would be spinning thefiber directly onto a PEM by utilizing a copper frame with an aluminumbacking. The PEM would be placed on top of the aluminum backing and heldin place with a copper frame that comes into contact with the perimeterof the PEM. PTFE masking could also be utilized in this type of fixturewhere electrical insulation is needed. Once the fiber is seeded onto themembrane, a catalyst ink or electrode could be applied to themembrane/fiber. The invention allows the production of a compositeelectrode where the fiber serves as both the reinforcement and as thematrix for the catalyst.

The catalyst ink could be electrostatically co-sprayed with the ionomerfiber. The catalyst particles would be attracted to thepositively-charged fiber via electrostatic attraction to form anelectrode (e.g. the anode). It should be noted that the catalyst inkcould alternatively be used without an ionomer binder. It should also benoted that the catalyst could be substituted with PtRu/C, Pt-black,PtRu-black, and other precious/non-precious metal catalysts. Single- andmulti-walled carbon nanotubes could also be used in these formulationsto boost electrical conductivity. It should further be noted that theelectrocatalyst could be encapsulated within the ionomer fiber duringthe electrospinning process by including the electrocatalyst in theinitial ionomer solution formulation.

The catalyst ink could be sprayed (may or may not be electrostatic) inconcert with fiber formation (parallel operation), after fiber formation(series operation) or a combination of both. Once the anode is formed,the spraying of the catalyst ink could be halted so that the PEM couldbe processed entirely by electrospinning/electrospraying. Once the PEMis processed to a suitable mat thickness (˜1 to 7 mils), the spraying ofthe catalyst ink could be re-engaged to produce another electrode (e.g.the cathode). This process could be performed in an iterative fashion tofabricate a continuous stack of MEAs with a single, high-aspect ratiopolymer electrolyte fiber (PEF). This technique could also be used forfabricating additional layers of the MEAs, such as catalyst supportstructures, gas diffusion media and bipolar plates.

The general parameters for electrospinning or electrospraying inaccordance with the present invention are defined through a series ofranges shown in Table 2. The data points taken through these rangesdemonstrated generally linear relationships for the polymers beingtested. The polymers tested included the following set forth in Table 3aand 3b, and are representative of the classes of polymers which may beused in accordance with the present invention, and as identified above.

It should be noted that both of these approaches can be used for MEAs ineither direct methanol fuel cells or PEM fuel cells. Furthermore,various processing schemes, and combinations thereof, could be used tofabricate the MEAs including hot pressing, painting, and spraying. Theseschemes, when utilized properly, could lead to graded compositionalporosity through the PEM, catalyst layers and GDL.

Single-Cell Testing

All MEAs were protonated in a 0.5M H₂SO₄ acid mixture at roomtemperature (23 C) for 2 hours followed by a deionized water rinse atroom temperature for 2 hours to remove residual acid prior to fuel celltesting. Various other protonation procedures and conditions could beused, such as elevated temperature and concentration. Single-sided ELATmaterials (E-tek DeNora) were used as gas diffusion media.

Single-cell testing was performed using a 600 W Fuel Cell Technologies,Inc. test station. This station is equipped with an Agilent Technologies120 A load module, digital mass flow controllers, an automated backpressure system, 5 cm² fuel cell hardware, an on-board AC impedancesystem and humidity bottle assemblies. The on-board electrochemicalimpedance spectroscopy system was utilized to measure the in situ highfrequency resistance (HFR) of each MEA at a frequency of 2 kHz. The HFRis the sum of the membrane, interfacial and electrode resistances.

All MEAs were conditioned at 80 C, 100 percent relative humidity (RH) at0.50 V for at least 2 hours before polarization curves were collected.Polarization curves were collected from 1.00 to 0.00 V at 0.05 Vincrements with a 30 second delay.

Characterization

Additional characterization (i.e. cyclic voltammetry and atomic forcemicroscopy) was performed on the MEAs fabricated from the ionomer bindersolution formulations. Details regarding these experimental techniquesare presented here.

Cyclic Voltammetry. Linear Sweep Voltammetry at 5 mV per second withdilute hydrogen at 30 psig cell pressure was performed with a SolartronAnalytical SI 1287 Electrochemical Interface. The test was run asdescribed in the Handbook of Fuel Cells—Fundamentals Technology andApplications, vol. 3, pp. 545-562. The resulting ECA was calculatedbased on the following equation.

${{ECA}\left\langle \frac{{cm}^{2}\mspace{11mu}{Pt}}{g\mspace{14mu}{Pt}} \right\rangle} = \frac{{charge}\mspace{14mu}{for}\mspace{14mu} H_{2}\mspace{11mu}{adsorption}\text{/}{desorption}\left\langle \frac{\mu\; C}{{cm}^{2}\mspace{14mu}{electrode}} \right\rangle}{\left( {210\frac{\mu\; C}{{cm}^{2}\mspace{14mu}{Pt}}} \right) \times {catalyst}\mspace{14mu}{loading}\;\left\langle \frac{g\mspace{14mu}{Pt}}{{cm}^{2}\mspace{14mu}{electrode}} \right\rangle}$

Atomic Force Microscopy. Tapping mode atomic force microscopy (AFM) wasperformed with a Digital Instruments Dimension 3000 scanning probemicroscope with a Nanoscope IV controller. A tapping mode tip made ofetched single crystal silicon with a nominal tip radius of curvature of5-10 nm was used during scanning. All samples were kept under desiccantfor 24 h prior to analysis. The samples were then scanned immediately atroom temperature within a 5 μm² sample area.

Results and Discussion

The electrochemical performance of a fuel cell is typically determinedby analyzing a polarization curve (cell potential versus currentdensity) as shown in FIG. 5. This curve shows a typical fuel celloperation where numerous irreversible losses contribute tooverpotentials which cause the cell potential to drop significantlybelow the theoretical (ideal) value of 1.23 V at 25 C as determined bythe Nernst equation. The same holds true for the polarization lossescompared to the open circuit voltage during an experimental run. Voltagelosses differ between theoretical and experimental equilibrium cellvoltages due to the cathode mixed potential between O₂ reduction and H₂oxidation from crossover to Pt/C. The initial decrease is associatedwith the activation polarization region where reaction rate losses atthe electrocatalyst dominate due to the sluggish reaction kinetics andlow catalyst activity. This is followed by a linear drop in cellpotential due to resistance losses (i.e. ohmic polarization). Resistancelosses are a combination of the resistance to the flow of electronsthrough the electrodes and interconnects and the resistance to the flowof ions through the electrolyte. The catastrophic drop in voltage athigher current densities is termed concentration polarization and isgenerally due to mass transport limitations of reactants to thecatalytically-active sites.

Since the focus of this invention is on the enhancement of componentsderived from two different MEA fabrication techniques, 1) theformulation of a novel ionomer binder solution and 2) the development ofnovel, high aspect ratio ionomer fibers to be used as precursors forelectrode and MEA fabrication, the following description is devoted todiscussing the results of each.

Ionomer Binder Solution Formulation. Representative polarization curvescomparing an electrode according to the invention (t-butyl alcoholformulation) to the current state of the art (n-butyl acetateformulation) are shown in FIGS. 6 and 7. FIG. 6 captures thepolarization region down to 0.4 V, which makes it relatively easy todiscern the resulting overpotential over a range of cell potentials.FIG. 7 shows a close-up of the activation polarization region. If oneuses 0.70 V as a benchmark for comparison between the electrodes, it canbe seen from FIG. 6 that the Battelle electrode (t-butyl alcoholformulation) has a current density of 0.6760 A per cm² with a highfrequency resistance (HFR) equal to 0.06 Ω·cm² while thestate-of-the-art electrode (n-butyl acetate formulation) has a currentdensity of 0.3996 A per cm² with a HFR equal to 0.08 Ω·cm². If one uses0.85 V as a benchmark for comparison between the electrodes, it can beseen from FIG. 7 that the Battelle electrode (t-butyl alcoholformulation) has a current density of 0.0631 A per cm² (HFR equal to0.06 Ω·cm²) while the state-of-the-art electrode (n-butyl acetateformulation) has a current density of 0.0288 A per cm² (HFR equal to0.08 Ω·cm²). These results may seem surprising since the same catalystloadings and pressing conditions were used. However, the solvent systemcan have a significant impact on the electrode structure and MEAperformance prior to the final MEA fabrication step. In order tounderstand this further, one must first understand the three-phaseinterphase.

Catalytic sites within fuel cell electrodes maintain a three-phaseinterphase to be effective. This interphase allows for electronic andionic continuity while providing access to fuel or oxidant. See FIG. 8for a detailed schematic of a single catalytic site at the cathode. FIG.9 shows a field-emission scanning electron microscope (SEM) image of anactual Pt catalyst supported on carbon black (Pt/C) electrode, whichcomprises the continuity phase. The larger open areas provide access forfuel or oxidant, which is the second phase. The ionomer binder andmembrane, which serve as the third phase, are absent from this photo sothat the Pt/C structure would be more apparent. It is desirable tosatisfy all three conditions for as many catalytic sites as possiblethrough electrode engineering to offer improvements in MEA performance.

Nafion® forms one of three states when mixed with organic solvents: (i)solution, (ii) colloidal dispersion, and (iii) precipitate. A truesolution is formed when the dispersed phase is molecularly dispersed,whereas in a colloidal dispersion the dispersed phase, or colloid, islarger than the molecule. The colloids are characteristic of dimension,however, they are also evenly dispersed throughout the dispersion medium(i.e. solvent). A typical scale for colloids is between 1 and 1,000 nm.As these colloids become larger they will either rise to the surface orfall out of solution forming a precipitate depending on the relativespecific gravities. For Nafion-based polymers and certain solvents undercertain conditions, these states can be classified by the dielectricconstant, ∈. The dielectric constant is used as a rough indication ofthe solvent's polarity. Some guidelines for Nafion®-based systems are asfollows:

-   -   When ∈ is greater than 10, a solution is formed.    -   When ∈ is greater than 3, but less than 10, a colloidal        dispersion is formed.    -   When ∈ is less than 3, precipitation occurs.

Uchida et al, Journal of the Electrochemical Society, vol. 142, page 463(1995) experimented with a variety of solvents and found that n-butylacetate (∈=5.01) was the best performance-enhancing dispersant agent dueto its ability to form a colloidal dispersion. The proposed mechanismthat was suggested was that Nafion® filled the macropores increasing thePt/ionomer contact area. At higher Nafion® loadings, however, theionomer hindered the rate of oxygen transport and the rate of waterremoval.

It was also found in the Uchida et al. publication that all solventsystems with dielectric constants greater than 10 formed solutions andresulted in MEAs that performed poorly in a fuel cell environment.However, t-butyl alcohol was not utilized in this study. Tert-butylalcohol is a water and alcohol soluble material with a boiling point of82.9 C, which is noticeably lower than butyl acetate (126.3 C), and hasa dielectric constant, where ∈=12.47 at 25 C and 10.9 at 30 C.

Furthermore, it is shown hereinbelow in FIGS. 6 and 7, that t-butylalcohol offers a pronounced improvement in fuel cell performancecompared to the n-butyl acetate system, which is contrary to the work ofUchida et al. It should also be noted that the high-frequency resistance(HFR) dropped from 0.08 to 0.06 Ω·cm². Thus, the improved electrodestructure also reduced the resistance/impedance throughout the MEA. Moregenerally, electrodes prepared according to the invention may haveimproved performance as measured by a current density of at least about0.6 A/cm² at a cell potential of 0.7 V with a high frequency resistanceless than 0.08 Ω·cm², under the conditions shown in FIG. 6. Theelectrodes may have improved performance over time as measured by acurrent density of at least about 0.6 A/cm² at a cell potential of 0.7 Vwith a high frequency resistance less than 0.08 Ω·cm², and a currentdensity of at least about 0.04 A/cm² at a cell potential of 0.85 V witha high frequency resistance less than 0.08 Ω·cm².

In some embodiments, membrane electrode assemblies prepared according tothe invention provide a surprising durability benefit. When an electrodeof the assembly is produced using a true solution according to theinvention, this has the effect of improving the durability of thepolymer electrolyte membrane of the assembly. FIG. 10 shows a membraneof an MEA according to the invention having superior durability comparedto the state of the art in an open circuit voltage experiment. In apreferred embodiment, the invention provides a membrane electrodeassembly wherein at least one of the electrodes is prepared from acomposition including an ionomer and at least one solvent selected asdescribed above, the membrane of the assembly having improved durabilityas measured by an open circuit voltage (OCV) holding time of at leastabout 100 hours under the conditions shown in FIG. 10, and preferably atleast about 300 hours. More generally, the invention provides a methodof improving the durability of a second component of an electrochemicaldevice by preparing a related first component of the electrical devicefrom a true solution according to the invention. The invention alsoprovides a method of improving the durability of a component of anelectrochemical device by preparing the component from the true solutionaccording to the invention. More generally, the invention provides amethod of improving one or more properties of an electrochemical deviceor its components by preparing at least one component of theelectrochemical device with a true solution according to the invention.The invention can apply to any types of components, such as membraneelectrode assemblies, membranes, electrodes and/or gas diffusion layers.

Beyond high performance and durability, another typically preferredproperty of an electrode structure is high catalyst utilization. Todetermine this, the in situ ECA within the fuel cell was measured bycyclic voltammetry using the area under the hydrogenadsorption/desorption peaks. These results are shown in Table 4.Surprisingly, the t-butyl alcohol system produced an electrode structurewith a significantly reduced ECA range compared to the n-butyl acetatesystem. In other words, the electrode according to the invention has alow catalyst utilization while still having an improved performance. Inone embodiment, the electrode has a low catalyst utilization as measuredby an electrochemical area of less than about 40 square meters ofcatalyst per gram of catalyst, sometimes less than about 35 squaremeters of catalyst per gram of catalyst, and the electrode has improvedperformance as measured by a current density of at least about 0.6 A/cm²at a cell potential of 0.7 V with a high frequency resistance less than0.08 Ω·cm².

Since the efficiency of catalyst utilization at high current density isdictated by mass transfer, which is governed by pore and ionomerdistribution as well as the oxygen concentration gradient through thethickness of the electrode, it is conceivable that these novelelectrodes enhanced the competing requirements for proton, reactant gasand water transport. Although it was apparent from these results thatthe rate of oxygen transport may have been hindered, water removal musthave been enhanced to allow such performance improvements. Thisenhancement may have led to the increased polarization at high currentdensity, where the catalyst layer resistances usually dominate. Byimproving the distribution of hydrophilic materials in the vicinity ofthe catalyst, it may help to draw water away from the active sitesthereby improving water transport and reducing localized flooding (i.e.optimizing water management and reducing mass transport losses).

AFM was utilized to examine the surface morphology of the electrodes.Tapping mode amplitude and phase images of the electrodes were recordedunder ambient conditions on a 5 μm×5 μm scale in order to investigatethe relative differences in surface morphology of the materials as shownin FIGS. 11 and 12. In FIG. 11, it can be seen that there are relativelylarge polymeric islands or domains present which disrupt the surfacemorphology and thus the continuity of the three-phase interphase in then-butyl acetate system. These islands range from about 0.63 μm to aslarge as 2.50 μm in length. From FIG. 12, however, it can be clearlyseen that the t-butyl alcohol system offers an electrode morphology thatappears to be closer to a percolation threshold compared to the n-butylacetate system, where there is more of an optimum balance anddistribution between the components that provide ionic transport, oxygendiffusion, water transport and an electrochemically active surface area.This also allows greater adhesion to the membrane substrate due to thepossible increase in polymer surface area that comes in contact with themembrane. These islands or domains are typically smaller than 0.63 μm inany one direction. In one embodiment of the invention, the electrode hasa morphology that includes domains of the polymer, where at least about80% of the polymer domains are smaller than about 0.63 μm in any onedirection, and preferably substantial all of the polymer domains aresmaller than this size. The polymer domains may be present on thesurface of the electrode and/or they may be present throughout theelectrode.

The solubility of the blends can change during electrode drying becauseof the difference in evaporation rates of the individual components.Azeotropic mixtures, a mixture of two or more liquids that has aconstant boiling point, can aid in this process.

Some advantages of the present invention may include: increased energydensities and efficiencies; reduced polarization losses; and reducedcatalyst loadings. In addition, advantages during MEA fabrication mayinclude: enhanced solvent removal (due to vapor pressures of thesolvents); paintability; faster processing due to enhanced evaporationrate; more precise control of catalyst loadings (layer-to-layerconsistency); enhanced electrode adhesion, and no need for viscositymodifiers (e.g. glycerol).

High Aspect Ratio Ionomer Fiber Development. Two different electrodeswere fabricated for this study. One included the high aspect ratioionomer fiber while the other did not. A representative polarizationcurve comparing these is shown in FIG. 13. FIG. 13 captures thepolarization region down to 0.4 V, which makes it relatively easy todiscern the resulting overpotential over a range of cell potentials. Ifone uses 0.65 V as a benchmark for comparison between the electrodes, itcan be seen from FIG. 13 that the Battelle invention (with ionomerfiber) has a current density of 0.545 A per cm² with a HFR equal to 0.09Ω·cm² while the one without an ionomer fiber has a current density of0.395 A per cm² with a HFR equal to 0.06 Ω·cm².

It is believed that a highly engineered electrode structure was producedwhich enabled the simultaneous and efficient access of protons,electrons and fuel to catalyst sites by incorporating a continuous,non-woven, high aspect ratio ionomer fiber mat within the electrode.This mat enhanced MEA performance by creating a multifunctional,nanostructured architecture, which reduced polarization losses. Thisionomer fiber provided a proton conducting pathway to the catalystsites, which created an intimate interphase with the electrocatalystwhile maintaining contact with the membrane of like materials. Finally,the polymer membrane and catalyst layers likely had commensuratedimensional changes upon hydration, thermal cycling, etc., which led toreductions in the overall internal resistance within the MEA causing anincrease in performance. This process also has advantages gained byforming fiber mats on the surface of conventional membranes to increasetheir effective electrode area while matching coefficients of thermalexpansion and water uptake rates between the electrode and the membrane,which minimizes stresses throughout the MEA.

The present invention may allow the production of PEM-based electrodesthat function at >100° C. and <50% RH.

The invention may also allow the production of a direct methanol fuelcell (DMFC) electrode and electrode assembly with decreased MeOHcrossover, higher operating MeOH concentration, decreased flooding andincreased durability.

As described above, in one embodiment of the invention, the electrodeand the polymer electrolyte membrane are both made with the same type ofionomer fiber. This embodiment provides several advantages, such as nomismatch of coefficients of thermal expansion, and thus minimizedresidual stresses throughout the MEA. An improved MEA is provided wherethe electrodes and PEM can shrink/swell in concert with each other.Electrodes are produced that now match higher temperature, lowerhumidity performing polymer membranes made with materials as describedabove

Some other advantages of the invention may include a highly-tailorableelectrode in regards to solid-state and surface chemistry to optimizethe interphase. The electrode may be mechanically tough with the abilityto impart flexibility and blunt cracks. The electrode may have improvedwater management to behave similarly to the Nafion®-alternative PEM. AnMEA prepared according to the invention may have improved mechanical andelectrostatic bonding.

In accordance with the provisions of the patent statutes, the principleand mode of operation of this invention have been explained andillustrated in its preferred embodiments. However, it must be understoodthat this invention may be practiced otherwise than as specificallyexplained and illustrated without departing from its spirit or scope.

TABLE 1 Solvent and Diluting Agent Properties Hydrogen SolubilityParameter Dielectric Solvent Bonding (MPa^(0.5)) Constant ethyl acetatem 18.6 6.02 methyl ethyl ketone m 19 18.4 ethylene glycol m 19.4 —monobutyl ether dichloromethane m 19.8 8.9 acetone m 20.3 20.7 dioxane m20.5 2.2 N-methyl-2- m 23.1 33 pyrrolidone N,N-dimethyl m 24.8 37formamide dimethyl sulfoxide m 29.7 46.7 chloroform p 19 4.8acetonitrile p 24.3 37.5 t-butyl alcohol s 21.7 12.4 1-butanol s 23.317.5 isopropyl alcohol s 23.5 19.9 benzyl alcohol s 24.8 13.1 ethanol s26 24.55 methanol s 29.7 32.6 glycerol s 33.8 42.5 water s 47.9 78.4n-butyl acetate m 17.4 5.01 N,N-dimethyl m 22.1 37.8 acetamide2-methoxyethanol m 23.3 16.93 hexane p 14.9 2.0

TABLE 2 Electrospinning/Electrospraying Condition Ranges Numerous datapoints exist Maximum Minimum through the range Distance to 4 cm 12 cmYes Target Orifice 22 gauge 18 gauge Yes Diameter Electrical 5 kV 30 kVYes Potential Flow Rate 0 mL/hr 0.26 mL/hr Yes Orifice blunt tip balltip Geometry Polarity positive negative

TABLE 3a Trial Data Concentration Viscosity Trial Polymer Range (wt %)(Pa · s) 1 BPSH45 15-45 0.01-0.03 2 BPSH35-6F30 25-25 3 Nafion ®  5-25

TABLE 3b Trial Data Surface Viscosity Tension Conductivity Trial ShearRate (s⁻¹) (dynes/cm) (μS/cm) 1 10³-10⁴ 35-45 700-1600 2 3

TABLE 4 Electrochemical Area Ranges ECA range Sample (m² Pt/g Pt) 28 wt.% t- 27.7-31.2 butyl alcohol 28 wt. % n- 43.3-47.4 butyl acetate

What is claimed is:
 1. A component for a fuel cell which is preparedfrom a composition comprising a true solution, wherein said truesolution is made of an ionically conductive polymer and at least onesolvent, the polymer and the at least one solvent being selected so that|δ solvent−δ solute|<1, where δ solvent is the Hildebrand solubilityparameter of the at least one solvent and δ solute is the Hildebrandsolubility parameter of the polymer in units of MPa^(0.5).
 2. Thecomponent defined in claim 1 which is an electrode for a fuel cell,wherein the composition further comprises a catalyst.
 3. The componentdefined in claim 1 which is a membrane for a fuel cell.
 4. The componentdefined in claim 1 which is a membrane electrode assembly for a fuelcell comprising a polymer electrolyte membrane having electrodes on bothsides, at least one of the membrane and the electrodes prepared from thecomposition.
 5. The component defined in claim 1 wherein the ionicallyconductive polymer comprises a perfluorosulfonic acid polymer.
 6. Thecomponent defined in claim 1 which is an electrode for a fuel cell,wherein the composition further comprises a salt, an oxide or a metalalloy.
 7. The component defined in claim 1 wherein the true solution isa polymer binder solution, the polymer in the solution functioning as abinder when combined with a catalyst.
 8. The component defined in claim1 which is a catalyst ink that additionally comprises a catalyst alongwith the ionically conductive polymer and the at least one solvent. 9.The component defined in claim 1 which is an electrode for the fuelcell, the electrode having a morphology that includes domains of thepolymer, at least about 80% of the polymer domains being smaller thanabout 0.63 μm in any one direction.
 10. The electrode defined in claim 9wherein all of the polymer domains are smaller than about 0.63 μm in anyone direction.
 11. The electrode defined in claim 9 wherein themorphology includes the polymer domains throughout the electrode.
 12. Amembrane electrode assembly for a fuel cell comprising a polymerelectrolyte membrane and a pair of electrodes on opposing sides of themembrane, wherein at least one of the electrodes is the componentdefined in claim 1, the membrane electrode assembly having improveddurability as measured by an open circuit voltage (OCV) holding time ofat least about 100 hours.